Audiometer and method of audiometry



y 3, 1956 J. ZWISLOCKI 2,753,397

AUDIOMETER AND METHOD OF AUDIOMETRY Filed Jan. 9, 19 52 INTENSITY CONTROL Q8 11 r INTENSITY CONTROL AMPLIFIERW 36 10 AMPLlF|ER T I I 26 2 34 51am 4 27' 3 RETIFIER United States Patent 2,753,397 AUDIOMETER AND METHOD OF AUDIOMETRY Joseph Zwislocki, Cambridge, Mass.,

A. G., Basel, Switzerland Application January 9, 1952, Serial No. 265,591 Claims priority, application Switzerland January 15, 1951 6 Claims. (Cl. 179--1) This invention relates to audiometry.

In acoustic audiometry, effected with electro-acoustic devices known as audiometers, each ear is tested individually and it must be ensured that the sounds presented are perceived only by the ear under test.

In testing the air conduction, the separation of the perception in both cars is automatic in most cases because in the first place the earphone applied to the ear under test transmits the sound directly to the ear drum, and in the second place because special packings at the earphone prevent the transmission of sound to the outside and to the ear not being tested.

In cases where the hearing acuity of both ears differs heavily this sound insulation is insufficient to prevent, during the testing of the poorer ear, the perception of the test sound by the better ear. In such cases a masking noise, to cover the test sound, must be fed to the better ear. A low intensity of noise is sufiicient to preclude the perception of the test sound by the masked ear.

During a test of bone conduction such a masking is indispensable. Being usually applied to the process of the temporal, the bone telephone produces in the entire skull a vibration which is imparted in practically equal degree to both ears and which as the intensity increases steadily will be perceived by the better ear first. Therefore, the ear not being tested can be shut off only by being masked.

A masking noise is characterized by the frequencies of which it is composed and by their intensities. It is known that masking noises of constant intensity are unsuitable because they may mask the test sound in the car under test. Audiometers in which the intensity of the masking noise is adjustable by hand give better results butare impractical. A practical solution necessitates an automatic regulation of the masking noise, the intensity of which is to increase and decrease with the intensity of the test tone. Only this method gives reliable results.

As to the frequency composition of the masker, two types are known so far:

1. Broad-band masking noise;

2. Pure tone masker.

The broad-band masking noise is composed of the greatest possible number of tones of the whole audible band so that its frequency spectrum represents a band ranging from the deepest to the highest audible tones.

The broad-band masking noise is said to have the advantage of being effective in conjunction with test tones of all frequencies. This advantage, however, is offset by the following facts:

0. To mask a certain test tone a noise is sufficient the frequencies of which are adjacent to that of the test tone. All other frequencies increase only the loudness of the masking noise in an undesired manner.

b. The intensity range between the threshold of hearing (minimum intensity) and the threshold of feeling (maximum intensity) depends on the frequency of the sound. Therefore, where a broad-band masking noise is used certain frequencies may reach the threshold of feeling assignor to Jaquet ice before the intensity necessary to mask the test tone has been achieved.

0. The calibration of the masking noise is difficult be cause the threshold of hearing for a broadband noise is not necessarily the same as for the test sound.

The masking with a pure tone consists in that the ear under test and the ear to b..- masked are fed with tones of the same frequency. To distinguish the test sound from the masking sound, the test sound is interrupted periodically. The disadvantage of that method resides in the fact that the patient tends to confuse the test sound and the masking sound. Moreover, a pure tone has a very small covering effect with respect to masking another tone of the same frequency so that undesirably high intensities are necessary for an effective masking.

The present invention eliminates the disadvantages of the known methods of masking with broad-band noises or with pure tones by using a narrow-band masking noise which is composed only of frequencies adjacent to the frequency of the test sound. The noise spectrum may be continuous or composed of single frequencies.

The narrow band noise masks only one test sound at a time and its mean frequency must be adjusted to the frequency of the test sound. The difficulty can be readily overcome by producing the masking sound through the amplitude modulation of a part of the electrical oscillation generating the test sound, and feeding the modulated oscillation to the masking earphone. It is known that such a modulation creates a frequency spectrum composed of the carrier and the side bands located symmetrically with respect to the carrier frequency and reflecting the frequency spectrum of the modulating signal. If the upper limit frequency of the signal is sufficiently low, a narrow band spectrum is created. its mean frequency is always the frequency of the carrier; accordingly, if the oscillation generating the test sound is used as carrier, the mean frequency of the modulated oscillation and with it the mean frequency of the masking noise changes automatically with the frequency of the test sound. The change of the mean frequency does not affect the relative location of the side bands.

Based upon this feature, the invention provides an audiometer comprising an electrical audiofrequency generator with variable frequency, means to deliver the generated electrical oscillation in desired magnitude to an electroacoustic transducer converting it into the test sound, an amplitude modulator modulating a part of said electrical oscillation with a low frequency signal and means to deliver the modulated oscillation in desired magnitude to an earphone converting it into a narrow-band masking noise.

Various kinds of amplitude modulators may be used for the transformation of the electric test sound oscillation into the narrow band masking noise, provided that all frequencies outside the desired band are suppressed to such a degree that they do not disturb the test procedure. It has been found, however, that the masking improves if the test sound frequency is eliminated from the noise spectrum, and, therefore, a carrier suppression modulator is preferred. Such modulators are known as push-pull and ring modulators.

The best modulating signal is random noise passed through a low-pass filter. In this case the most efficient band width of the masking sound is, as termed in the literature, a critical band. However, a 50 or 60 C. P. S. frequency derived from electric mains may be considered as sufficient under most circumstances. It likely constitutes the most economical modulating signal. The masking noise can be improved distorting the 50 or 60 C. P. S. signal in a nonlinear network. Circuits containing varistors seem to be adequate for this purpose.

The described narrow-band masking noise has approxi- '5 mately the same threshold as the test sound and appears suited for accurate calibration and automatic adjustment to the intensity of the test sound. In order to achieve such an automatic adjustment it is sufficient to maintain the electrical energ supplied to the masking earphone proportional to the energy delivered to the test sound transducer. This can be realized within certain limits by deriving the carrier for the modulation from a circuit place having an electrical oscillation proportional to that supplied to the test earphone. A greater variability can be obtained, however, by using a twin attenuator for the test sound and the masking noise electrical currents.

In the accompanying drawing an embodiment of the invention is shown by way of example.

The figure is the diagram of an audiometer.

In the figure, I is a device to produce the test sound, II an accessory which is electrically connected with that device and serves to produce a masking noise. The device I comprises a rectifier 3 which is connected to the mains terminals 2 and in which the operating voltages required for the audiometer are produced. This rectifier 3 has connected to it a resonant circuit, which comprises a thermionic tube 4 operated at the desirable working point by way of a cathode resistance 5. The frequency of oscillation is determined by the coil 6 and a manually adjustable capacitor 7. The feed-back is effected by a coil 8, and in a coupling coil 9 the voltage to produce the test soundthe so-called test sound voltage-is induced.

This test sound voltage is amplified in a conventional thermionic amplifier 10 and is fed by way of an intensity control 11 to an earphone 12, in which the alternating electric voltages are converted into sounds. The earphone 12 is applied to the patients eat 13 to be tested, or, for testing the transmission by the bones, to the process of the temporal behind the ear 13. The person under test should not perceive with the other car 14 the test sound of the ear phone 12. To this end the accessory II is provided, which has connected thereto the masking earphone 15, which is applied to the ear 14 and in which a masking noise is produced. The alternating voltages necessary to produce this noise are generated in a ring modulator of known type, which is composed of four thermionic tubes 24, 25, 26, 27, and four resistances 28, 29, 30, 31. mentioned above the mean frequency of the masking noise should vary automatically with the adjustable frequency of the test sound, the ring modulator is fed with the unamplified test sound voltage which, acting as a carrier, is tapped at the points 16 and 17. By way of the capacitors 18 and 19 and the resistors 20, 21, 22, and 23, acting as voltage dividers, the voltage is fed to the ring modulator. The modulating voltage may be taken from the mains because a frequency of 50 C./S. to 60 C./S. sufiices. For this reason it is taken at 32 and 33 from the mains and is fed to the resistors 30 and 31 directly or by way of a voltage distortion device 34. The output voltage of the ring modulator is applied by way of the transformer 35 to the thermionic amplifier 36. The intensity at the amplifier output is controlled by the intensity control 37. The same is coupled by a mechanical device 38 with the intensity control 11 so that both controls 11 and 37 are always operated simultaneously. From the intensity control 37 the voltages are supplied by way of the additional potentiometer 47, which enables the masking noise intensity to be varied independently of the test sound, to the masking earphone, where they produce the masking noise acting on the car 14.

Since for the reasons I claim:

1. An audiometer comprising an electrical audiofrequency generator of variable frequency, means to deliver the generated electrical oscillation in desired magnitude to an electroacoustic transducer converting it into the test sound, an amplitude modulator modulating a part of said electrical oscillation with a low frequency signal and means to deliver the modulated oscillation in desired magnitude to an earphone converting it into a narrow band masking noise.

2. An audiometer comprising an electrical audiofrequency generator of variable frequency, means to deliver the generated electrical oscillation in desired magnitude to an electroacoustic transducer converting it into the test sound, a carrier-suppression modulator modulating a part of said electrical oscillation with a low frequency signal and means to deliver the modulated oscillation in desired magnitude to an earphone converting it into a narrow band masking noise.

3. An audiometer comprising an electrical audiofrequency generator of variable frequency, means to deliver the generated electrical oscillation in desired magnitude to an electroacoustic transducer converting it into the test sound, an amplitude modulator modulating a part of said electrical oscillation with a low frequency signal derived from a common A. C. power source and means to deliver the modulated oscillation in desired magnitude to an ear phone converting it into a narrow band masking noise.

4. An audiometer comprising an electrical audiofrequency generator of variable frequency, means to deliver the generated electrical oscillation in desired magnitude to an electroacoustic transducer converting it into the test sound an amplitude modulator modulating a part of said electrical oscillation with a low frequency signal derived from a common A. C. power source and distorted in a nonlinear circuit and means to deliver the modulated oscillation in desired magnitude to an earphone converting it into a narrow band masking noise.

5. An audiometer comprising an electrical audiofrequency generator of variable frequency, means to deliver the generated electrical oscillation in desired magnitude to an electroacoustic transducer converting it into the test sound, an amplitude modulator modulating a part of said electrical oscillation with a low frequency signal, a generator to produce the low frequency signal and means to deliver the modulated oscillation in desired magnitude to an earphone converting it into a narrow band masking noise.

6. An audiometer comprising an electrical audiofrequency generator of variable frequency, means to deliver the generated electrical oscillation in desired magnitude to an electroacoustic transducer converting it into the test sound, an amplitude modulator modulating a part of said electrical oscillation with a low frequency signal and means to deliver the modulated oscillation to the masking earphone in a magnitude approximately proportional to the magnitude of the electrical oscillation fed to the test sound transducer.

References Cited in the file of this patent UNITED STATES PATENTS 2,232,779 Fletcher Feb. 25, i941 2,235,733 Witting Mar. 18, 1941 2,287,401 Wengel June 23, 1942 

